A typical hydraulic rack and pinion power steering gear for a vehicle includes a rack which extends axially through a chamber inside a hydraulic cylinder. The ends of the rack project axially outwardly from the terminal ends of the hydraulic cylinder and are connected with steering linkage which connects to steerable vehicle wheels.
A piston is fixed to the rack within the chamber. Rotation of the vehicle steering wheel actuates a hydraulic valve and causes hydraulic fluid under pressure to act against the piston. The force exerted by the hydraulic fluid moves the piston within the chamber, causing the rack to move axially. The axial movement of the rack moves the steering linkage which, in turn, causes the steerable wheels to turn.
To prevent hydraulic fluid from flowing out of the chamber, it is necessary to seal the ends of the chamber through which the rack projects. It is also desirable to support the rack at least at one end of the chamber for axial movement within the chamber. Finally, it is common to enclose the ends of the chamber with bellows attached to the hydraulic cylinder to prevent the ingress of moisture, dirt, or other contaminant.